This invention relates generally to a freeze protection control device utilized in a solar hot water heating system having a collector and associated conduits that are exposed to potential freezing temperatures. Specifically, the system includes and utilizes both ambient and collector panel temperature sensors which monitor for freezing temperatures and collector and tank temperature sensors which provide a differential that establishes useable available solar energy. The system includes an override circuit that prevents drain down even when ambient temperatures are well below freezing if sufficient useable solar energy is available to allow additional energy to be gained by the system.
In many present day systems using solar energy absorption collectors, it is common to heat water, either potable or nonpotable and as heated, transfer the water to a hot water storage tank inside a building or dwelling or the like. The collector and associated conduits are necessarily exposed outdoors. One of the problems in using such a system is that whenever solar energy is not available, the temperature of the fluid medium such as water in the collector and conduits which are exposed outdoors, will often times in northern climates drop below freezing, causing damage to the system. One solution to prevent collector freeze up in the past has been to provide a temperature sensor in the collector so that whenever the collector temperature approaches freezing, drain down of the collector is actuated. One of the problems, however, is that by the time the collector temperature approaches freezing, other components, conduits, or accessories such as a vacuum breaker or drain conduit had already frozen which would then prevent drain down of the collector, resulting in a freeze up of the collector. Thus, sensing collector temperature alone is not adequate to prevent freeze up. However, if ambient temperature alone is utilized to drain the system, there will be a large loss of useable available energy in that often times solar energy is available even though ambient temperatures are well below freezing. On a cold, sunny winter day in a northern climate, a solar collector can work quite efficiently to heat water for the storage tank in spite of the fact that the outside air temperature is below freezing. Utilization of an ambient sensor alone would not allow capturing of this available solar energy and, more importantly, would not protect the collector panels from the phenomenon of "night sky radiation" whereby on clear nights the temperature of a solar collector may drop as much as 20 degrees F. below ambient temperatures. Other present day systems often use heated water from the storage tank to prevent freeze up, thereby wasting energy already gained.
The present invention overcomes the deficiencies found in the prior art freeze protection systems by providing for ambient temperature sensing which controls the drain down valve but which includes a drain down override circuit which prevents drain down based on a predetermined differential temperature between the collector and the storage tank which indicates that solar energy is available to allow the system to continue in operation regardless of ambient temperature. The invention also allows simultaneous monitoring of collector and ambient temperatures, thus also protecting the panels from freezing due to "night sky radiation".